Synthesis gas (hereinafter referred to as syngas) is a mixture of hydrogen (H2) and carbon monoxide (CO). Syngas can be produced, in principle, from virtually any feedstock material containing carbon. Carbonaceous materials commonly include fossil resources such as natural gas, petroleum, coal, and lignite. Renewable resources such as lignocellulosic biomass and various carbon-rich waste materials can also be used to produce syngas. It is preferable to utilize a renewable resource to produce syngas because of the rising economic, environmental, and social costs associated with fossil resources.
There exist a variety of conversion technologies to turn these various feedstocks into syngas. Conversion approaches can utilize a combination of one or more steps comprising gasification, pyrolysis, steam reforming, and/or partial oxidation of a carbon-containing feedstock.
Syngas is a platform intermediate in the chemical and biorefining industries and has a vast number of uses. Syngas can be converted into alkanes, olefins, oxygenates, and alcohols. These chemicals can be blended into, or used directly as, diesel fuel, gasoline, and other liquid fuels. Syngas can also be directly combusted to produce heat and power.
U.S. Pat. No. 4,752,623 (Stevens and Conway) discloses a catalyst for producing mixed alcohols from syngas, wherein the catalyst contains either molybdenum or tungsten, in addition to either cobalt or nickel, both components being in sulfided form. Stevens and Conway emphasize that it is not necessary for their invention that any particular stoichiometric metal sulfide be present. Sulfided cobalt is often assigned to CoS in the literature. Further, Stevens and Conway state that no advantage is realized by the presence of sulfur in the feed.
U.S. Pat. No. 4,675,344 (Conway et al.) describes a method for controlling the ratio of methanol to higher alcohols by adjusting the concentration of a sulfur-releasing compound in the feed to a reactor containing alkali-promoted MoS2 catalysts. Conway teaches that such catalysts should exclude Group VIII elements, such as cobalt, to realize the selectivity benefit of sulfur addition.
Murchison et al. discuss the use of cobalt-molybdenum sulfides for mixed-alcohol synthesis in volumes 2 (pages 626-633) and 5 (pages 256-259) of the Proceedings of the 9th International Congress on Catalysis (1988). Murchison stated that cobalt (or nickel) molybdenum sulfide materials could be operated without added H2S. On page 257, Murchison stated that “[t]he effect of H2S in increasing the chain length of the alcohols is very specific for the alkalinized moly sulfide, both supported and unsupported. The addition of cobalt to the catalyst results in higher molecular weight alcohols without H2S addition and makes the catalyst selectivity substantially independent of the H2S of the feed . . . .” During the discussion at the 9th International Congress on Catalysis, as recorded at page 258, a questioner commented that “Santiestaban et al. indicate carbides are not formed” and poses a question, in response to which Murchison stated “[t]here is certainly no problem in long term stability without H2S if one is not concerned with maximizing C2+ alcohols. The XRD and XPS work we have done has not shown any carbide formation for the sulfided catalysts.” Murchison teaches that Co/Mo sulfides will be stable for alcohol synthesis in the absence of H2S and that carbides do not form as the catalyst acquires operating time on stream.
The existing art provides little, if any, information concerning chemical or physical characteristics that tend to correlate with the performance of cobalt-molybdenum-sulfide alcohol-synthesis catalysts, including Co—Mo—S, and similar catalyst systems comprising Ni and/or W. Particularly absent is information relating to preferred amounts of sulfur, on a stoichiometric basis, relative to other major components present.
Furthermore, the existing art does not provide guidance with respect to maintain catalytic stability (in sulfided systems) for long periods of time. Yet, catalyst lifetime and stability (i.e., the maintenance of activity and selectivity) are critical from a commercial point of view, for economic reasons.
In light of these shortcomings in the art, what are needed are methods of making preferred sulfided catalyst compositions, methods of using these catalyst compositions, and methods of maintaining sufficient activity and sulfide content to convert syngas into alcohols, such as ethanol.